The present invention is related to an electrolytic capacitor. More specifically the present invention is related to an electrolytic capacitor comprising intrinsically conductive polymeric cathode layers capable of achieving a high break down voltage (BDV) which were not previously available with polymeric cathode layers.
Solid electrolytic capacitors with intrinsically conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageous low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. Intrinsically conductive polymer, more commonly known as conductive polymer, is electrically conductive in the molecular level. In other words, a single molecule (a polymer chain) of this type of polymer is conductive, which distinguishes itself from other groups of polymeric materials whose electrical conductivity is imported from the presence of foreign conductive particles. The example of the latter is polyester (non-conductive) filled with carbon back (conductive particles). The intrinsically conducting polymer can exist in many physical forms including solid, solution, and liquid dispersion.
The backbone of a conductive polymer consists of a conjugated bonding structure. The polymer can exist in two general states, an undoped, non-conductive state, and a doped, conductive state. In the doped state, the polymer is conductive but of poor processability due to a high degree of conjugation along the polymer chain, while in its undoped form, the same polymer loses its conductivity but can be processed more easily because it is more soluble. When doped, the polymer incorporates anionic moieties as constituents on its positively charged backbone. In order to achieve high conductivity, the conductive polymers used in the capacitor must be in doped form after the completion of processing, although during the process, the polymer can be undoped/doped to achieve certain process advantages.
Various types of conductive polymers including polypyrrole, polyaniline, and polythiophene are described for use in Ta capacitors. The major drawback of conductive polymer capacitors, regardless of the types of conductive polymers employed, is their relatively low working voltage compared to their MnO2 counterparts. Since their introduction to the market, the working voltages of Polymer Ta capacitors has been limited to 25 V, while the working voltages of Solid Ta capacitors (MnO2 cathode) available on the market can reach 75 V and the working voltage of Wet Ta capacitors can reach 150V. This limitation has made applications of polymer Ta capacitors in high voltage circuits impossible which is where the combination of low ESR and non-burning failure mode are most critical.
During manufacture the Ta powder is mechanically pressed to make Ta metal pellets. The pellets are subsequently sintered at high temperature under vacuum. The sintered anodes are then anodized in a liquid electrolyte at elevated temperature to form a cohesive dielectric layer (Ta2O5) on the anode surface. Increasing formation voltage increases the dielectric thickness, which determines the maximum voltage the anodes can withstand. Polymer cathodes are conventionally applied to tantalum capacitors by synthesis from the monomer and an oxidizing agent. This is known as ‘in-situ’ polymerization. Typically the anodes are prepared by the steps of dipping in oxidizing agent, drying, dipping in monomer, reacting the monomer and oxidizing agent to form conductive polymer and washing of byproducts not necessarily in this order. Optionally, a reform step may be applied after washing to reduce DC leakage of finished capacitors.
With reference to FIG. 2, there is a large increase in leakage current at about 35 V for both capacitors comprising a polymeric cathode (In-Situ Ctrl and In-Situ Test) despite the formation voltage being 125 V. This can be compared to MnO2 cathode, which does not show an appreciable increase of leakage current until about 70 V, and the wet, sulfuric acid, cathode which does not show an appreciable increase in leakage current until about 120 V. The dielectrics for both the MnO2 and wet devices were also formed to 125 V. Thus, despite the high formation voltage, it would be difficult to rate polymer capacitors above about 25 V.
After formation of the polymer coating graphite and silver are applied to allow adhesion to the cathode lead. The manufacturing process is then continued by assembling, molding and tested the capacitors.
The rating voltage for Ta capacitors, or the working voltage allowed for reliable operation, is primarily a function of dielectric thickness. Dielectric thickness is controlled by the formation voltage. Increasing the formation voltage increases the dielectric thickness. It is estimated that for every volt applied during the dielectric formation process, about 1.7˜2 nm of dielectric is formed on the surface. For a given anode, increasing dielectric thickness is at a cost of capacitance loss since the anode capacitance is inversely proportional to dielectric thickness. It is a common practice for solid Ta capacitor manufacturers to use a formation voltage which is 2.5 to 4 times higher than the anode rated voltage. This ensures high reliability during applications. For example, a 10V rated capacitor often employs an anode formed at 30V.
A plot of the BDV versus the formation voltage for a wide range of Ta capacitors including both polymer (polyethyldioxythiophene, or PEDOT) and MnO2 based capacitors is shown in FIG. 1.
As shown FIG. 1, in the low formation voltage region (≦30V), the BDV of both polymer and MnO2 capacitors are close to the anode formation voltages. However, there is a trend of divergence in terms of BDV between MnO2 and polymer capacitors as formation voltage increases from about 80V to 200V. In this range, while the BDV of MnO2 parts still increases with increasing formation voltage, the BDV of polymer capacitor shows a mostly flat pattern. This has been interpreted in the art to indicate a limit of about 50V which is almost unaffected by the increasing formation voltage. Increasing dielectric thickness, which is the most important and commonly used approach to make high voltage capacitors, is virtually ineffective for making high voltage polymer capacitors beyond about 25V ratings. Due to this phenomenon the Ta industry has had difficulty producing reliable conducting polymer capacitors for use above 25 V. A 35V rated capacitor, for example, would require a BDV of far greater than 50V to ensure its long term reliability (e.g. 35V rated MnO2 parts have an average BDV of 95V. While not limited to any theory it is postulated that the polymer/dielectric interface can cause the differences in BDV.
In recent years conductive polymers have received considerable attention. This material is a suspension of conductive polymer in a solvent. Instead of the conventional method of applying the conductive polymer by in-situ synthesis from the monomer and an oxidizing agent, the polymer can now be applied by dipping in the slurry and removing the solvent. Again with reference to FIG. 2 the leakage current vs. voltage behavior of the slurry polymer cathode compares favorably to the in-situ formed cathode. A significant improvement is obtained. Large leakage currents do not flow until about 75 V. Thus, devices of 35 V ratings can be manufactured. However, even with these polymer slurry cathodes, leakage current performance is still unsatisfactory. Wet tantalum devices with formation voltages of 125 V would be expected to be rated up to 70 V. Thus, a significant improvement in rated voltage of tantalum capacitors with polymer cathodes is still needed if they are to compete with wet tantalum capacitors in higher voltage ratings.
It is known in the art, that reducing the oxygen and carbon content of the anodes leads to the formation of a better quality dielectric. It has been demonstrated on tantalum capacitors with wet and MnO2 cathodes that reduction of oxygen and carbon contents can significantly improve the long term reliability of the capacitors. However, little improvement is noted in the initial performance. This is show in FIGS. 3a and 3b for a tantalum capacitor with a MnO2 cathode. It would not be expected that reduction of oxygen and carbon contents of the anode would improve the initial performance of tantalum capacitors with polymer cathodes. Indeed, applying these state of the art processing techniques in combination with conventional methods of depositing conductive polymer leads to only a very small improvement in performance. FIG. 1 shows that applying these techniques leads to only a few volts improvement in where the leakage current increases as indicated in the ‘in-situ test’. The combination of polymer deposited by an in-situ method and the best anode technology still leads to a device that cannot compete in rated voltage with MnO2 or wet capacitors. Thus, these capacitors still fall short of meeting the goal of replacing MnO2 or wet capacitors with a lower ESR device at rated voltages above 25 V.
There has been a long standing desire in the art to provide a capacitor comprising a conducting polymeric cathode suitable for use at higher rated voltages. Through diligent research the present inventors have achieved what was previously not considered feasible.